Carrier modulation



Jan. 29, 1946. 1 M. LEEDS 2,393,785

CARRIER MODULATION Filed Nov. 3, 1942 2 Sheets-Sheet 1 7 l VARIED ATMODULATING FREQUENCY CARRIER summon 9 1 LOAD VARIED AT Z MODULATING yFREQUENCY Fig.3.

11/ ll/ T H l l 9') VARIED AT RRIER *gQ MODULATING B i LOAD. *7

REQUENCY Laurence PL Leeds,

His Attorney.

Jan. 29, 1946.

q. M. LEEDS CARRIER MODULATION F iled Nov. 3, 1942 2 Shets-Sheet 2 Invent or" w e emaw L m M Wo e m cxMA n 8 mm u t Patented Jan. 29, 1946UNITED 'STATE s PATENT 1 OFFICE v CARRIER MODULATION Laurance M. Leeds,Rotterdam Junction, N. Y,

assignor to General Electric Company, a corporation of New YorkApplication November 3, 1942, Serial No. 484,341

Claims. (01. ire-171.5)

generator to its load through an impedance matching transformer having aratio varying at modulating frequency to control the carrier output.With such an arrangement the circuit etliciency is independent of themodulation percentaKe.

The object of my invention is to provide an improved and more efllcientarrangement for modulating a carrier wave, I

The novel features which I believe to be characteristic of my inventionare set forth with par-,

ticularity in the appended claims. My invention itself, however, both asto its organization and method of operation, together with furtherobjects and advantages thereof, may best be under- 1 stood by referenceto the following description taken in connection with the accompanyingdrawings in which Figs. 1 to 4 inclusive represent carrier modulationcircuits; Figs. 5 to 8 inclusive. represent networks for obtaining avariable inductance through the use of a variable capacity; Figs. 9 to12 inclusive represent networks for obtaining a variable capacit throughthe use of a variable inductance; and Fig. 13 represents a variablecapacity having its value determined by the instantaneous modulatingpotential applied to its control electrode.

Referring to the drawings, in Figs. 1 to 4 inclusive there is shownasource of radio frequency power I having output terminals 2 coupledthrough reactance networks consisting of-quarter wavelength artificialtransmission lines I, I, I, and 8 to a load, such as an antenna,represented by a resistance 1. The reactance networks are made up ofinductances I and condensers s which are varied in magnitude atthemodulating frequency to produce a variation in the coupling betweenthe power source and its load, thereby modulating the power output.

In operation the reactances of the inductances and condensers can bevaried so that the voltage across the load resistance 1 is given by theequation e,=E'.[1+m cos (pt)] cos (wt) 1 u is the carrier frequencyangular velocity. It

will be recognized by those skilled in the art that Equation 1 is theenvelope equation of a distortionless modulated signal.

The manner in which this result is obtained is explained in connectionwith Fig. 1 for the case in which the reactances of the inductances Iand condensers 0 are equal. For this condition the input impedance ofthe network, the impedance at the terminals 2, is given by the equationR. where Z is the input impedance, X is the react-F ance of the networkcondensers and inductance at the R. F. carrier frequency, and R. is theload resistance 1,

Because the modulation network 3 is made up of reactances, all of thepower fed into the network must appear in theload resistance I. Fromthis it follows that the power input is equal to the power appearing inthe resistance 1 where e. and 2!:- are, respectively the instantaneousand maximum voltages across the load resistance 1, m is the modulationpercentage p is the modulating frequency angular velocity, d

equation Q I z--- 3 where en, the instantaneous voltage across terminals2 of the R. F. power source is equal to E0 cos .wt. From Equations 1, 2,and 3 the instantaneous value of the reactances in the network is givenby the equation 0. E.(l+m cos pt) cos wt ('4) From Equation 4 isapparent that, in order to obtain the distortionless modulated signaldefined by Equation 1, itis necessary that the reactances in themodulation networks vary at modulation frequency in accordance with theequation constant 1+m cos pt (5) equal to zero and the networkreactances are infinite.

Because the modulation networks use both variable condensers andvariable inductances and because it may be more convenient to use onlyone type of variable reactance, the circuits shown in Figs. 5 to 12inclusive will be useful in practical modulation networks.

The circuits of Figs. 5 to 8 inclusive are quarter wavelength artificialtransmission lines made up of fixed condensers l and inductanoes ithaving variable condensers i2 connected across output terminals l3which, due to the circuit constants, appear at the input terminals H asvariable inductances.- Any one of the networks shown in Figs. to 8 maybe substituted for the variable inductances in the modulation networksof Figs. 1 to 4 inclusive and will provide at the terminals ll anapparent inductanc given by the equation where L is the apparentinductance, X is the reactance of the fixed inductanoes and condensersin the networks, and C is the instantaneous capacity of variablecondenser i2.

The circuits of Figs. 9 to 12 inclusive are made up of fixed condensersl5 and inductances I! having output terminals l1 shunted by variableinductances l8 which, due to the circuit constants, appear as variablecondensers at the input terminals IQ of the network. Any of the circuitsshown in Figs. 9 to 12 inclusive may be substituted for the variablecondensers in the modulation networks of Figs. 1 to 4 inclusive andprovide at the terminals i an apparent capacity given by the equation Iwhere C is the apparent capacity, X is the reactance of the fixedcondensers and inductances in the network, and L i the instantaneousinductance of the variable inductances l8.

In Fig. 13 is shown a variable condenser of the construction disclosedin Patent 2,243,829, Brett et al., which is capable of modulationfrequency variation necessary for operation of the modulation networksalthough the range of variation is not suillcient for 100 per centmodulation. This condenser comprises an evacuated envelope 2| containingspaced condenser plates 2i traversed by a beam 22 of electrons from anelongated cathode 23. The electrons from the cathode pass through slitsin a control electrode 24, in an accelerating electrode 25, and in ascreen suppressor electrode 26, and are collected on a plate 21. Thescreen suppressor electrode 22 shields the condenser plate 2! from thevoltages applied to escapes the electrodes 24 and 2| and also preventsthe return of electrons from the plate 21. The beam of electrons passingbetween the condenser plates varies the dielectric constant and, sincethe beam is controlled by the potential e. applied to the controlelectrode 24, the capacity appearing at the condenser terminals 22 isdirectly related to the potential at the control electrode 24. Due tothe inertialess nature of the electron beam, the eflective capacityappearing at the terminals 28 can be varied at a high frequency.

The instantaneous value of the capacity at the terminals 22 is given bythe equation o t X E.(l +m cos pt) (9) From this it follows thatJBef'EJl-l-mc (11 (10) 4 (1E. e JZ[I+mCOB (111)] If the relation betweena; and the capacity of the terminals 22 is linear, the modulation willbe distortionless if e; is distorticnless.

, Neglecting the small amount of power required to control thereactances I and 9, the efliciency is the instantaneous efllciency ofthe power source, i. e., the eillciency of the power source at theparticular loading existing at that instant, and is independent of themodulation percentage. The

' power source may be an oscillator, power amplifier, or other deviceand will have an efficiency dependent upon its design. If the powersource is a class C amplifier, its instantaneous efficiency can be veryhigh and in general will be in excess of 70 per cent at any modulationpercentage.

The transmission apparatus disclosed above is applicable to anymodulated transmitter and should result in a very simple high powerbroadcast or television transmitter.

While I have shown particular embodiments of my invention, it will beunderstood that many modifications may be made without departing fromthe spirit thereof, and I contemplate by the appended claims to coverany such modifications as fall within the true spirit and scope of myinvention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. Transmission apparatus comprising a source of carrier, a quarterwavelength artificial transmission line, made up of variable series andshunt reactances, for connecting the carrier output with a load, andmeans for varying simultaneously each of said reactances at modulatingfrequency to modulate the carrier output while maintaining constant theelectrical length of the line.

2. In a modulation system, a source of oscillations of constantamplitude, a load, a reactance network between said source and load,said network having both shunt and series reactanoes and an electricallength equal to an odd multiple of a quarter of a wavelength at thefrequency 01 said source, a source of modulating voltage, and means tomodulate the voltage across said load in accord with said modulatingvoltage, said means comprising means to modulate both the shunt andseries reactances of said network in accord with said modulating voltagewhile maintaining constant said electrical length of said network.

3. In a modulation system, a source of oscillations of constantamplitude, a load, a reactance network between said source and load,said network having both shunt and series reactances and an electricallength equal to an odd multiple 01 a quarter of a wavelength at thefrequency of said source, a source of modulating voltage. and means tomodulate the voltage across said load in accord with said modulatingvoltage, said means comprising means to vary both the shunt and seriesreactances of said network in accord with the product 01' a constant and(1+m cos at) where m is the percentage modulation, 1: is the frequencyof said modulating voltage, and t is time.

4. In a modulation system in which a source of constant voltage ofcarrier frequency is supplied through a reactance network to a load,said network having an electrical length equal to a quarter or awavelength or odd multiple thereof at the frequency of said source, themethod of modulating the amplitude of voltage supplied to said loadwithout modulating the phase thereof which comprises varying thecharacteristic impedance of said network while maintaining constant theelectrical length of the network.

5. In a modulation system, a source of carrier wave oscillations, aload, a reactance network connected between said source and load. saidnetwork comprising series reactance of one sign and shunt reactance ofopposite sign and having an electrical length equal to an odd multipleor a quarter of a wavelength, a plurality of capacitance devices, eachcorresponding to one of said reactances and each having capacitance ofvalue corresponding to modulating voltage applied thereto, means toapply thereto modulating voltage in accord with which it is desired tomodulate said carrier wave oscillations supplied to said load and meansto modulate each of said positive and negative reactances in accord withthe corresponding one 01' said capacitances thereby to vary the voltagesupplied to said load, said positive and negative reactances beingvaried in such relation as to maintain constant the electrical length ofsaid network between said source and load.

LAURANCE M. LEEDS.

